A plurality of architectures for mobile radio transmitters are known which can be produced with little complexity. Particularly in the case of radio transmitters which operate using time slots, “bursts”, it is appropriate to use “open loop modulation”. In this case, a phase locked loop first locks onto the desired transmission frequency. Prior to the actual transmission of useful data, the control loop is opened and the carrier is modulated with the modulation signal while the control loop is open. However, this principle has the drawback that drift effects can cause the carrier frequency to change in open operation, which means that specifications cannot be observed any longer. This can be prevented using relatively complex stabilization devices.
It is also known practice to use “two-point modulation” as an alternative to the implementation of a radio transmitter concept for modern mobile radio systems. Two-point modulation involves the modulation signal being fed into a phase locked loop at two injection points. Of these two injection points for the control loop, one has high-pass characteristics and one has low-pass characteristics. While the modulation signal is being fed in, the phase locked loop remains closed in this case. Accordingly, the carrier's drift effects which arise during open loop modulation are avoided. Advantageously, two-point modulation allows the transmission of modulation signals whose bandwidth is greater than the actual modulation bandwidth of the phase locked loop. The actual bandwidth of the control loop can thus be of relatively small design, which in turn significantly improves the noise characteristics of the arrangement. In this case, a two-point modulator can be produced with relatively little complexity.
A two-point modulator of this type is specified in the printed document DE 199 29 167 A1. This document shows a phase locked loop (PLL) into which a modulation signal is fed both at a high-pass point at the input of the oscillator and at a low-pass point in the feedback path of the PLL.
In principle, there are a plurality of conceivable options for selecting the injection points for the modulation signal into the control loop. By way of example, the modulation signal may be supplied in digital form to a frequency divider in the feedback path of the phase locked loop, in which case this frequency divider normally operates as a fractional N divider. At the same time, the same modulation signal, but in analog-coded form, is fed in at an input on the oscillator in the phase locked loop, said oscillator normally being in the form of a voltage-controlled oscillator.
In the case of such two-point modulation with a combination of digital and analog modulation, it should be remembered that the analog and digital modulation signals are in phase and that there is a good match between the amplitudes of these two signals. Manufacturing tolerances (which are unavoidable in the case of mass production) in the device components, which are of significance to analog modulation and, by way of example, influence the modulation gradient, the modulation voltage generation etc., mean that it is necessary to perform amplitude trimming between the analog and the digital modulation following production of a radio transmitter of this type. If the intention is also to take into account temperature influences on the parameters of the devices used, it is desirable for such trimming to be carried out not just once during production but also prior to every transmission operation.
One option for performing this trimming is to receive a transmitted, modulated signal using an additional measuring receiver, to demodulate it, to measure the modulation swing and to perform trimming using the data obtained in this manner. The trimming information then needs to be stored in a memory in the appliance itself, however. In addition, this allows only production-related, but not temperature-related or aging-related, drift effects to be taken into account.
Another option might be to use the reception part of a transceiver to trim the transmission part. However, this would have the associated drawback that it would be necessary to provide an additional phase locked loop which, in the case of a heterodyne receiver, would need to be set to a frequency which corresponds to the differential frequency between the transmission frequency and the intermediate frequency.